1. Field of the Invention
The present invention relates to an image writing device, and particularly to a light source unit for an image writing device.
2. Prior Art of the Invention
A multifunctional printer having functions such as of a copier, a scanner, a printer and a facsimile machine is equipped with an image writing device writing image data such as letters, graphics and electrophotographs stored in a storage medium such as a hard disk onto a original sheet.
In the image writing device, as shown in FIG. 18, a LSU (a laser scanner unit) has been conventionally employed. The LSU has a structure in which a light source 104 emits light on the basis of image data on a specific scan line to acquire a light beam, and the light beam is exposed to a specific face of a polygon mirror 102 rotating at specific rotation speed to expose a light reflected at a reflection angle in a specific range to a photosensitive drum 101 through a f·θ lens 103.
In the image writing device equipped with the LSU, a necessity arises for accelerating the rotation speed of the polygon mirror 102 in proportion to a resolution or a printing speed of an image. In a case where, for example, an image data of 600 dpi (24 dots/mm) is printed at 200 mm/sec, a necessary rotation speed of the polygon mirror 102 having six reflective faces results in 24×200×60/6=48,000 RPM.
Considering a load received by bearings for the polygon mirror 102, a noise generated by rotation of the polygon mirror 102, however, it is not preferable to accelerate the rotation speed of the polygon mirror 102.
Thus, a configuration that a LED array 110 in which a number of LED elements 111 are arranged on a LED chip 112 as shown in FIG. 19 is arranged so as to oppose to a rod lens 113 as shown in FIG. 20 has been commonly employed in recent years. In this configuration, the light emitted from the LED array 110 is exposed to a photosensitive drum 101 through the rod lens 113.
In a case where the LED array 110 is adopted as a light source, a PN junction structure is required for each of the LED elements 111 in order to control light emission by flowing a current; therefore in turn requiring a specific space between adjacent LED elements 111. In a light source for an image writing device capable of printing an image of, for example, 600 dpi, LED elements of about 20 μm square are arranged with a pitch of about 42.3 μm.
While in order to print a high resolution image, necessity arises for a smaller spacing between the LED elements 111, a technical problem is raised in reducing a necessary space for accommodating a component to control light emission of an LED element 111. Therefore, the space has to be ensured by decreasing a size of the LED element 111 itself, but such down-sizing of an LED element 111 entails impossibility of acquiring a sufficient luminance for forming a latent image on a photosensitive member. In order to attain a sufficient luminance, a high voltage may be imposed on an LED 111 element, but with the result of adverse reduction in lifetime of the LED element 111.
In a conventional image writing device, a light source for one pixel was formed with one light emitting element (for example, an LED element 111). Therefore, in order to compensate dispersion of luminance in light emitting elements, correction means performing shading correction or the like was necessary for an image writing device. Dispersion of luminance include initial dispersion caused by a size dispersion generated in fabrication of light emission elements and uneven emission efficiencies of LED elements 111 and uneven change on standing in luminance of LED elements 111 according to respective use states thereof.
There exists a phenomenon that an luminance at the periphery of an area illuminated with light on a specific surface is lower than at a central portion thereof. For this reason, in a case where a light source of one pixel is formed with one light emitting element, an illuminance at the periphery has a chance to be lowered as compared with that at the central portion, in an area corresponding to each pixel illuminated with a light beam. Hence, the periphery of each pixel is lighter on a print than the central portion thereof, having lead to a problem that a sharp image is unable to be printed.
In order to down-size an image writing device, a method has been available in which a distance from an LED element 111 to the photosensitive drum 101 (hereinafter referred to as a conjugate distance) is shortened. In order to reduce a conjugate distance, a diameter of each rod lens 113 has to be decreased.
As the diameter decreases, however, optical noise such as cross talk between rod lenses and flare light increases. That is, there has been a limitation on development in down-sizing of the device based on a prior technique.
In a case where a rod lens 113 is employed as a collective lens as in a prior art, a rod lens 113 has to be adopted with which a diameter of a spot light impinging for illumination on the photosensitive drum 101 is the same as a diameter of a light source (an LED element 111).